The FAA has received reports from the field of throttle and mixture control levers loosening during service, resulting in either a partial or complete loss of throttle and/ or mixture control. The subject throttle and mixture control levers were manufactured from a bronze material. Splines on the mating control shaft are pressed into the bronze control levers during installation by torquing a lock nut. Reinstalling bronze material levers can cause premature wear on the lever splines thereby reducing the amount of engagement with the control shaft. Additional wear during service can lead to complete disengagement of the splines and result in loss of throttle and/ or mixture control.

Bronze material is no longer used to manufacture these levers. CMI now manufactures these levers from stainless steel. The stainless steel levers have machined splines that are more durable to prevent premature wear. Additionally, CMI has issued CMI Critical Service Bulletin CSB08-3B, Revision B, dated February 1, 2013, which requires replacement of all bronze material throttle and mixture levers with the new stainless steel levers at the next 100 hour, or annual, inspection, or when the lever is removed for any reason, whichever occurs first.

The FAA recommends removal of the bronze material throttle and mixture control levers from service in accordance with the guidance in CMI Critical Service Bulletin CSB08-3B.